Strengthening glass by multiple alkali ion exchange



March 18. 1969 A, E SAUNDERS ET AL 3,433,61

s STRENGTHENING GLASS BY MULTIPLE ALKALI ION EXCHANGE Filed Sept. 9,1965 Sheet of 6 LAYEP. DEPTH IN MICEONSA IOO FGG. I

I' I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I II I I I I I I I I I I I I I I I I SRENGTHBNING GLASS BY MUTIPLE ALKALIION EXCHANGE Filed sept] 9, 1985 'aofG Sheet 5 ma zommmnou MGE mamTENSION PSI ISO

LAYEP. DEPTH IN MICIZONS INVENTORS AK//OLD E. SAUNDEES FIO. Z

ATTORNEYS March 18. 1969 A. E. sAuNDERs ET AL 3,433,6l1

STRENGTHENING GLASS BY'MULTIPLE ALKALI ION EXCHANGE Filed Sept. 9, 1955Sheet 3 of 6 SUEFACE COMPRESSION PSI TENSION PSI -zspoo lo zso 81s 500LYE DEPTH |N MICRONS INVENTORS Aww z. snwmes F' 'G- 3 .ms/e75. A'u/c/MN"Y ATTORNEYS March 18. 1969 -A. E. sAuNDERs ETAL 3,433,61I

STRENGTHENING GLASS BY MULTIPLE ALKALI ION EXCHANGE 4 ofe Sheet FiledSept. 9, 1965 k\ 0 O III'IYO m, w, ma w mm na zomwmzou mumaw TINVENTORS' LAYEB DEPTH IN MICRONS ov/ (v w .WM WM O ,Wmwm mw, D 460W 07mm .5 0W w 4 m F March 18. 1969 A E SAUNDERS ET AL 3,433,611

STRENGTHENING GLASS BY MULTPLE' ALKALI ION EXCHANGE Filed Sept. 9, 1965Sheet 5 of 6 SUEFACE COMPEESSION PSI TENSION PSI LAYEE DEPTH IN MICRONSINVENTORS FIO. 5 Ale/mo a. s/wNss a, ,9085/27 .5. kuB/CHN BY wk ATTORNEYMarch 18, 1969 A, E, SAUNDERS ET AL :BA-33,611

STRENGTHENING GLAss BY MULTIPLE ALKALI IoN EXCHANGE Filed sept. 8. 1985Sheet of e loopoo H sueFAczcoMPfeEssloN Psl Tenslon psI -zspoo I I I 20-lso 250 300 375 500 LAYER DEPTH IN MlcoNs INVENTORS FIG. 6 ARNoLo a.SAUNDERS kamera walc/MN w BY v lvwx TTORNEYS United States Patent O3,433,611 STRENGTHENING GLASS BY MULTIPLE ALKALI ION EXCHANGE Arnold E.Saunders, Saxonburg, and Robert E. Kubichan, Tarentum, Pa., assignors toPPG Industries, Inc., a corporation of Pennsylvania Filed Sept. 9, 1965,Ser. No. 486,126 U.S. Cl. 65-30 Claims Int. Cl. C03c 21/00 ABSTRACT OFTHE DISCLOSURE neous exchange of potassium and sodium ions wherein thepotassium ions are present in greater quantity than the sodium ions.

Ion exchange strengthening treatments are of two general types. Thefirst of these types is represented by U.S. 2,779,136, issued to Hood etal. This patent discloses the ion exchange of a relatively small sizeionic diameter alkali metal ion, such as lithium ion, from a molten Saltbath for a relatively larger size ionic diameter alkali metal ion, suchas sodium ion, from the glass. The exchange is conducted at atemperature above the strain point of the glass and results in alteringthe glass surface composition to a composition having a lowercoeificient of thermal expansion than the coeflicient of thermalexpansion of the interior glass composition. The difference in thermalexpansions of the surface and interior Zones results in a surfacecompressive Stress developing in the glass article when it is cooled toroom temperature.

The second general type of ion exchange treatment used to increase thestrength of glass is represented by Republic of South Africa Patent622,353, issued to Coming Glass Works. This ion exchange techniqueinvolves exchanging a relatively large ionic diameter alkali metal ionfrom a molten Salt bath for a relatively smaller ionic diameter alkalimetal from the glass at a temperature below the strain point of theglass. At this temperature, the Surface of the glass does not relax toaccommodate the larger sized ion being exchanged -into it. The result isthe production of a surface compressive Stress when the ion exchangedglass article is cooled to room temperature. The mechanism differs fromthat taught by Hood et al. in U.S. 2,779,136 in that it involvesmechanically wedging a larger ion into the hole left in the glasssurface structure by the removal of a smaller ion.

Ions other than the alkali metals ions have also been exchanged inglass. J. S. Turnbull and W. A. Weyl in their article entitled StainingGlasses with Silver, a Possibility of Studying Inhomogeneities (theGlass Industry, 1940), disclose ion exchanging silver ions for sodiumions in a glass article to detect inhomogeneities by observingdifferences in the amount of silver staining developed in various partsof the article.

U.S. 2,647,068, issued to Imre Patai, discloses a method of silverstaining a soda glass by contacting the glass with a molten salt bathcontaining a mixture of alkali metal ions and silver ions. The stainproduced results from the exchange of silver ions from the mixed bathfor sodium ions in the glass. No ion exchange of alkali metal ions fromthe bath for sodium ions in the glass is realzed. The exchange is alsonot noted to result in any glass article strength improvement. Althoughthe Patai bath contains a mixture of ions only the silver ionspreferentially exchange in the glass.

In our copending application Ser. No. 293,271, now abandoned, filed July8, 1963, by Dale W. Rinehart, there are disclosed several ion exchangestrengthening treatments for a family of lithia-soda and/orpotassiumphosphorous pentoxide-alumina-Silica glass compositions. One ofthe ion exchange treatments disclosed is the exchange of sodium from amolten Salt bath for lithium in the glass surface at a temperature belowthe strain point of the glass. A second ion exchange disclosed is theion exchange of potassium from a molten Salt bath for both lithium andsodium in the glass at a temperature below the strain point of theglass. Both techniques result in increased glass article strength.

The sodium for lithium ion exchange, disclosed by Dale W. Rinehart,results in a relatively thick surface compressive layer and a relativelylow maximum surface compressive Stress. The potassium for sodium andlithium ion exchange, on the other hand, results in a relatively highmaximum surface compressive Stress and a relatively thin surfacecompressive layer. Thin surface compressive layers such as thoseproduced in potassium for sodium and/or lithium exchanges, since theyare easily penetrated by surface scratches, result in poor abradedstrengths even though the surface layers produced are high incompressive stress. A high surface compressive Stress, though desirablebecause glass normally fails in tension from a surface initiated crack,is still inadequate to alone permt the production of commerciallyattractive glass articles. The surface compressive Stress layer mustalso be of suffieient thickness to permt the glass article to retain itsstrength when abraded.

The earlier ion exchange techniques permitted a choice between thedevelopment of a relatively thick low maximum Stress surface compressivelayer or a relatively thin ihigh maximum Stress surface compressivelayer. Both of the desirable conditions, a high surface compressiveStress and a relatively thick surface compressive layer could not beobtained by a Single ion exchange treatment.

What has been discovered in the present invention is a technique forobtaining both of these desirable conditions. More particularly, whathas been discovered is that by properly compounding the composition ofthe base glass being ion exchanged and the composition of the ionexchange bath, the advantage of each type of ion exchange can be realzedWhile minimizing the disadvantage of each. A family of glasscompositions has also been discovered which undergoes a low degree ofpreferential ion exchange when treated in accordance with the techniquesherein disclosed.

The calculated chemical compositional ranges of the components in theglasses found suitable for multiple, simultaneous ion exchange inaccordance with the present invention are presented below:

Component: Percent by weight Li20 2-15 Na20 and/ or K2O 2-20 P205 1-25A12O3 10-35 SiOz 30-65 ZnO 0-12 MgO 0-8 E203 0-10 Zr02 0-8 The preferredcalculaited compositional ranges for the various components arepresented below:

4 changed in a 100 percent potassium nitrate molten salt bath.

The present invention will be more fully understood Comznnt' Percent byWell ibynaking rfererlce to tfhe folowiigdexample: h t 'T xampe is t epre erre em o iment o t e presen 11:1 280 and/Or K20 5 invention and isthe best mode contemplated by the in- Azl'zos 15 28 ventor forpraoticing the teachings of his invention. SiOz 40-60 EXAMPLE I Zno o 3Glass samples 4 inches by 4 inches by 1/10 of an inch Mgo 0-4 thick werefabricated using conventional melting and E203 0-7 forming techniqueshaving the following calculated base Zr02 0`4 glass composition:

Various other oxide components such as CaO, BaO, composition A Si02 andPbO can be incorporated in these glasses in P b ht amounts not to exceedapproximately 2 percent by weight Oxlde.0mpnent' ernt y wg to modify theglass structure. 15611 (2) 26'61 Ti02 may also be present in theseglasses in amountS .2 3 not to exceed approximately 5 percent by weightto aid 1'120 5'04 in controlling the high temperature viscosity charac-Na20 teristics of the glasses. P205 Various glass coloring ingredientssuch as compounds Zno 3'0 containing iron, cobalt, nickel, gold, silver,chromium, The strain point of Composition A is approximately magnesium,copper, Selenium, platinum and graphite may 971 F. Samples of this glasswere then ion exchanged in also be added in small concentrations tocolor the glass mixed alkali metal molten salt baths by immersing thewithout impairing its desirable multiple and simultaneous glass samplesin the bath. Samples of this glass were also ion exchange properties.Components such as Sb205, ion exchanged in single alkali metal ionmolten salt baths As2O5, Na2SO4, NaCl and S may also be incorporated infor comparison purposes. amounts up to about 1 percent by weight. Theion exchanges were conducted in an apparatus con- Characteristicallymixed ion exchange baths are found sisting of a stainless steelcontainer provided with a built to exhibit preferential ion exchangeproperties. Usually in electrical resistance heater. The alkali metalsalt or only the most mobile ion specie is exchanged to any salts usedwere introdnced into the stainless steel consignificant degree even inbaths containing very low container and heated until the salts melted.No stirring centrations of the most mobile ion. As a general rule,mechanism was employed. The molten salts were homothe preferentiallyexchanged ion has the smallest atomic genized by natural convectioncurrents established by difdiameter of the exchangeable ions in thebath. ferential temperatures in the bath during heating.

In the present invention for the first time a mixed alkali Table Ipresented below compares the results of four ion exchange bath is usedto treat a family of glass comion exchange treatments on glassComposition A. The positions which does not result in a preferentialexchange ratios of KNO3 to NaN03 in the mixed baths indicated of onlyone of the exchangeable ions. The greater depth are molar ratios of KNO3-to NaNO3.

TABLE I Temper- Total Surface ature o Treating compression Compression,Molten Salt Ion Time Layer Depth Lbs. per

Exchange, Sq. In

850 200 mlcrons. 60, 000 KNOa 925 18 microns. 96, 600 KN 03+N aNOa, 8=875 215 microns 92, 000 KNO3+NaNOz, 20:1 875 -do 185 mlcrons 98, 900

to which the ion exchange is oonducted 'and the relatively high surfacecompressive stress achieved make possible the production of strongunique glass articles.

To aid in explaining the techniques of the present invention, FIGURES 1through 6 are provided in which:

FIGURE 1 is a concentration profile of various alkali metal ions in thesurface of a glass article treated in a mixed ion exchange bathcontaining a 20:1 ratio of KN 03 to NaNO3 on a molar basis as a functionof glass thickness;

FIGURE 2 is a stress profile determined for a glass article ionexchanged in a 20:1 potassium nitrate to sodium nitrate mole ratiomolten salt bath;

FIGURE 3 is a stress profile determined for a glass article ionexchanged in an 8:1 potassium nitrate to sodium nitrate mole ratiomolten salt bath;

FIGURE 4 is a stress profile determined for a glass article ionexchanged in -a 100 percent sodium nitrate molten salt bath;

FIGURE 5 is a stress profile determined for a glass article ionexchanged in a 100 percent potassium nitrate molten salt bath and FIGURE6 is a stress profile determined for a glass article that has first beenion exchanged in a 100 percent sodium nitrate molten Salt bath andthereafter ion ex- The 8:1 ratio mixed KNO3+NaNO3 bath was prepared bycombining 809 grams of KNO3 and grams of NaNOa. The 20:1 ratio mixedKNO3+NaNO3 bath was prepared by combining 1,000 grams of KNO3 and 42grams of NaNO3.

As can be seen from the table above, the percent sodium nitrate ionexchange bath developed a relatively thick compressive surface layer(200 microns) and a moderate surface compressive stress (60,000 poundsper square inch). The potassium nitrate ion exchange, on the other hand,Produced a much higher surface compressive stress (96,000 pounds persquare inch) but a very shallow compression layer of only 18 microns.The mixed bath containing both sodium ions and potassium ions developedboth a relatively thick compressive layer (215 and microns) and a highsurface compressive stress (92,000 and 98,900 pounds per square inch).

EXAMPLE II A sample of Composition A, 1/2 inch by l inch by 1/10 of aninch thick was fabricated using conventional melting and formingtechniques. This sample was ion exchanged in ia mixed ion exchange bathcontaining KNO3 and KaNO3. The ratio of KNO3 to NaNO3 was 20:1 on amolar basis. The sample was treated for 60 minutes at 875 F. After theion exchange treatment a flame spectrophotometric .analysis w'as made todetermine the potassium, 'sodium and lithium concentrations at variousdepths in the surface of the sample. These analytical results arepresented below in Table II.

TABLE IL-FLAME SPECTROPHOTOMETRIC ANALYSIS Alkali Moles per Mid-pointDetermined 100 Gr. Glass Etch Micron No. Depth Percent Percent Pereent KNa. Li

K Li

The concentraton of sodium 'in the base glass was 8.1 percent -by weightor .35 moles per hundred grams of glass. The concentraton of lithium inthe base glass was 2.2 percent |by weight or .31 moles per hundred gramsof glass. The base glass composition initially contained no potassium.

As can be seen from the analysis, the sodium concentraton is greaterthan the base glass sodium concentration from a depth of labout micronsto a depth in excess of 240 microns. The glass also contains a potassiumconcentraton from the surface to a depth of about 20 microns. Thelithium concentraton is below that of the base glass for a depth inexcess of 240 microns. The above analysis indicates that both potassiumand sodium have been ion exchanged into the glass and that lithium hasbeen removed.

FIGURE 1 is a plot of the lithium, sodium and potassium concentrationsin percent by weight and in moles in per 100 grams of glass at variousdepths in the glass sample. The exact mechanism of this multiple ionexchange is not understood.

FIGURE 2 is the stress profile determined for the 1/2 inch by 1 inch by1/10 of an inch sample of Composition A in Example II treated in themixed 20:1 potassium to sodium mole ratio ion exchange bath. The maximumsurface compressive stress occurs at the surface and is approximately96,000 pounds per square inch. This sur;

face compressive stress decreases rapidly as penet-ration is made intothe sample unit, at a depth of about microns, the stress has dropped toapproximately 14,000 pounds per square inch. At this depth 'an abruptchange in the rate at which the compressive stress i-s being lost,occurs. As can be seen in FIGURE 2, the rate of compressive stressrelaxation after the abrupt change is fairly constant but considerablydecreased. A slight residual compressive stress is still present 'in theglass at a depth of 250 microns. The abrupt change in the nate ofrelaxation of the surface compressive stress or the knee in the stressprofile curve occurs at approximately the same depth to which potassiumhas been exchanged. The knee also occurs at approximately thecorresponding depth at which the sodium concentraton in the glasssurface is a maximum.

ICompar'ing the chemical profile analysis and the stress profile, leadsthe applicant to believe that the portion of 6 the stress profile fromthe surface to a depth of about 25 microns represents the stress induced.in the glass surface by lthe exchange of potassium ion. The portion ofthe stress profile from the knee occurring at about 25 microns, to adepth of about 250 microns represents the stress induced by exchange ofsodium ion.

The interior of the glass, that portion at least 300 microns from eithersurface, is in tension. The maximum tensile stress developed in theinterior is approximately 2,000 pounds per square inch.

The typical article produced in the present invention by ion exchangingthe A composition in a mixture of sodium and potassium salts exhibitsthree stress zones. These three stress zones are a surface compressivestress zone, an intermediate compressive stress zone, and an interiortensile stress zone. The concentraton of lithium ion is greater in theinterior tensile stress zone than n both the surface compressive stresszone and the intermediate compressive stress zone. The concentraton ofsodium ion is greater in the intermediate compressive stress zone thanin both the central interior tensile stress zone and the surfacecompressive stress zone. The concentraton of potassium .ion is greaterin the surface compressive stress zone than in both the intermediatecompressive stress zone and the central interior tensile stress zone.

EXAMPLE III A sample of Composition A glass 1/2 inch by 1 inch by :A0 ofan inch thick was prepared using conventional melting and formingtechniques. The sample Was then ion exchanged in a mixed potassiumn'itrate-sodium nitrate bath prepared by combining 805 grams ofpotassium nitrate and grams of sodium nitrate. This mixed bath contained'a mole ratio of potassium to sodium of 8:1. The ion exchange wasconducted at a temperature oi 875 F. for 60 minutes.

FIGURE 3 is the stress profile determined for this sample. The maximumcompressive stress occurs at the glass surface, and is approximately96,000 pounds per square inch. The surface compressive stress decreasesrapidly as penetration is made into the glsas sample until, at a depthof about 25 microns, the surface compressive stress is approximately35,000 pounds 'per square inch. At this depth an abrupt change in therate of surface compressive stress relaxation is again noted. Thesurface compressive stress continues to decrease but at a much slowerrate as further penetration is made into the glass. Some compressivestress is found to remain at a depth of 250 microns. The interior of theglass, that -portion at least 300 microns from either surface, is intension. The maximum tensile stress developed in the interior beingapproximately 3,-'000 pounds per square inch.

EXAMPLE IV Samples yof glass '1/2 inch by 1 inch by 1/10 of an inchthick of Composition A and of Composition B indicated below wereprepared using conventional melting and forming techniques.

Composition B The strain point of composition B is -approximately of thesample, `a tube within the shield to conduct the 919 F. abradingparticles onto the unprotected portion of the Table III presents datadescribing the types of comglass sample and a funnel atop the conductingtube pressive surface layers developed for several single specie throughwhich the -abrasive particles were introduced. The alkali metal ionexchange treatments on glass Composir abrasion was accomplished byintroducing 'a jet of comtions A and B. pressed air into the conductingtube and adding the abra- TABLE III Ion Temper- Depth of Surface GlassExchange aturc of Time of Compressive Compressive Composition TrcatingExchange, Exchange Layer Stress Bath F. Developed Developed A NaNO3. 85090 minutes..- 300 microns. 55, 000 850 ...do 30 Inicrens. 112, 000 800.do 150 microns... 65,000 800 ..do 8 microns. 114,000

FIGURE 4 shows the stress profile determined for the sive particles tothe funnel. The air accelerates the abra- 1/2 inch by 1 inch by J/o ofan inch sample of glass comsive and drives it onto the surface of theglass sample. The position A treated in a 100 percent sodium nitratemolten abrasive used was 1A cubic centimeter of 100 B-Alumdum salt bathat 850 F. for 90 minutes. 20 having a particle size of approximately 250microns. The FIGURE 5 shows the stress profile determined for the gritwas driven onto the glass surface of the glass sample 1/z inch by 1 inchby 17/10 of an inch sample of glass Comby a jet of air -at poundspressure per square inch. position A treated in a 100 percent potassiumnitrate The grit was poured into the funnel by hand and took molten saltbath at 850 F. for 90 minutes. only a second or two to be driven throughthe conducting Table IV presents data describing the types of comtubeonto the glass sample. The abrasive removed appressive surface layersdeveloped for several mixed alkali proximately .001 of an inch of glass.The abraded area metal ion exchange treatments on glass Compositions Aappeared frosted due to light scatte'ring from the roughand B. Thecolumn entitled Ion Exchange Treating ened sur-face. Bat lists the moleratio of potassium nitrate to sodium The unabraded and the abradedsamples were then nitrate in the treating bath. tested to determinetheir modulus of rupture strengths TABLE IV KNO: tO

NaNO; Temper- Depth of Surface Ratio in ature of Time of CompressiveCompresslve Glass Composition Ion Exchange, Exchange Layer StressExchange F. Developed Developed Treating (microns) (p.s.i.)

Bath

EXAMPLE V using a concentric ring load testing apparatus. Each samplewas placed on a 3-inch-diameter support ring and a ll/z-inchdiameter-ring placed on top. The upper ring was positioned in theapproximate center of each sample, which in the case of the abradedsamples was also the approximate center of the abraded area. The loadwas applied to each sample by raising the bottom support ring at aconstant rate of speed against the fixed upper ring. The load wasapplied 'at a rate to develop a Stress in the sample of approximately10,000 pounds per square inch per minute. The modulus of rupturestrength in pounds per square inch was then calculated from the load atA glass sample '1/2 inch by 1 inch by 1/10 of lan inch thick ofComposition A was prepared using conventional melting and formingtechniques. This sample was subjected to a stacked ion exchangetreatment. The glass sample was first ion exchanged in a 100 percentsodium nitrate molten salt bath at 850 F. for 90 minutes. This. samplewas thereafter ion exchanged in a 100 percent potassium nitrate moltensalt bath at 875 F. for 30 minutes. FIGURE 6 shows the ;stress profiledetermined for Composition A subjected to the stacked technique justdescribed. A comparison of FIGURE 6 with FIGURES 1 and 2 faouro omg thefollowing formula' points out that the techniques of the presentinvention S=.463W/'l`2 permit the production of a strengthened glassarticle in a single ion exchange treatment which previously required at'least two separate ion exchange treatments.

where S equals the modulus of rupture in pounds per square inch, Wequals the failure load and T equals the thickness of the sample ininches.

EXAMPLE VI The results of Example VI are presented in Table V below: Todemonstrate the advantages of the present mventlon several samples ofComposition A, 4 inches by 4 inches TABLE V by 1/to of an inch were ionexchanged in various single KNozm and mixed ion exchange treating baths.The abraded (55 Sample Il Tra' Time of Mgflus 'and unabraded modulus ofrupture Strengths Were then No. Ion Exchange Exchange, Exchange Rupturedetermined for the samples to compare the several treat- Tflng F' 1'000(p's'i') ments. For each ion exchange treatment investigated, twosamples were prepared. One sample was then abraded. 12:""':jgfoz g oomgutef'" gfiged' The unabraded sample and the abraded sample were thenNaNoz only--- 850 'I 72 unabraded. tested to determine their modulus ofrupture strengths "do g 'z lzdrl'edm pounds 'per square inch. 880 28abraded.

The abrading apparatus consisted of a circular sample g support, atubular shield to confine the abrasion to a cir- 880 880 ---.do 23abraded.

culm' area about 3A of an inch in diameter in the center As can be seenin Table V above the 100 percent potassium nitrate ion exchangetreatment developed a very high unabraded modulus of rupture strength.The abraded strength, however, was extremely low (13,000 pounds persquare inch) due to the relatively thin surface compressivehlayerdeveloped. The 100 percent sodium nitrate ion exchange treatmentdeveloped a lower unabraded modulus of rupture strength than the 100percent potassium nitrate treatment, but retained a higher strength whenabraded. The higher abraded strength for the 100 percent sodium nitrateion exchange is due to the thicker compressive layer developed.

The glass samples exchanged in the mixed ion exchange baths are seen toexhibit abraded strengths approxmately double that of the 100 percentpotassium nitrate ion exchanged abraded strengths and higher unabradedstrengths than the 100 percent potassium nitrate ion exchange strengths.

Although the technique of the present invention has been described usingvarious mixtures of sodium nitrate and potassium nitrate, the inventionis not so restricted. Other salts such as the chloridcs and sulfates mayalso be used.

The present invention is also not limited to the alkali metals ofpotassium and sodium. Other alkali metals such as rubidium and cesiummay be combined with potassium or sodium or with mixtures of potassiumand sodium to produce unique glass articles. For example, a mixture ofrubidium, potassium and sodium salts can be used to produce a glassarticle having two knees in its stress profile.

A glass article vion exchanged in such a three alkali ion salt bathwould possess three distinct compressive stress zones. The threecompressive zones Would be first: a very high surface compressive stresszone, a second intermediate zone of high compressive stress and a thirdzone of moderately high compressive stress. These three compressivestress zones would also ditfer in thickness. The very high surfacecompressive stress zone would be very thin. The second intermeditaecompressive stress zone would be somewhat thicker and the moderatelyhigh compressive stress zone would be of substantial thickness. Theconcentrations of the various alkali metals in the three compressivezones would also be different. Rubidium would be in greatestconcentration in the outermot surface compressive stress zone. Thesecond intermediate compressive stress zone would contain the highestconcentration of potassium and the third compressive stress zone wouldcontain the highest concentration of sodium. The knees in the stressprofile would occur at the approximate depths to which potassium ion andrubidium ion were exchanged.

Depending upon the article desired, the ion exchange baths of thepresent invention may contain all combinations and mixtures of at leasttwo of the following alkali metals: cesium, rubidium, potassium andsodium. The stress profiles which can be developed may contain as manyas four separate knees.

It is also possible to ion exchange a glass article in accordance withthe teachings of the present invention by contacting the article with apaste mixture containing exchangeable alkali metal ions. The paste wouldbe applied to the glass article and then heated for a suflicient time atan elevated temperature but below the strain point of the glass to causethe desired ion exchange.

The preferred alkali metal salt bath mixtures of the present inventionconsist of various combinations of potassium nitrate and sodium nitrate.The preferred ratio of potassium nitrate on a molar basis, is between8:1 and 20:1. This preferred molar ratio range develops the desiredcombination of high surface compressive stress and depth of the surfacecompressive layer. The molar ratio of potassium to sodium can vary from2:1 to 50:1, however, and still result in multiple ion exchange in theglass compositions disclosed in the present invention.

Although the present invention has been described in 10v terms ofspecific examples, the scope of the invention should only be limited bythe language of the appended claims.

We claim:

1. A method of treating a glass article consisting essentially of thefollowing components, in percent by weight, 2 to 15 percent LzO, 2 to 20percent of an alkali metal oxide selected from the group consisting ofNa20, KzO and mixtures of Na20 and KZO, 1 to 24 percent P205, 10 to 35percent AlzOa, 30 to 65 percent SiOz, 0 to 12 percent ZnO, 0 to 8percent MgO, O to 10 percent B2O3, and 0 to 8 percent ZrOz, whichcomprises contacting the glass at a temperature below the strain pointwith a source of at least two exchangeable alkali metal ions ofdifferent ionic diameter larger than lithium, the molar ratio of largeralkali metal ions to smaller alkali metal ions in said source being atleast 2:1, to exchange simultaneously at least two of the alkali metalions of the source for alkali metal ions of the glass.

2. A method according to claim 1 in which the source of at least twoexchangeable different sized ionic diameter alkali metal ions of largerionic diameter than lithium is a molten mixture of potassium and sodiumsalts.

3. A method laccording to claim 2 in which the mixture is a mixture ofpotassium nitrate and sodium nitrate.

4. A method according to claim 3 n which the mixture of potassiumnitrate and sodium nitrate is a mixture containing at least a 4:1 ratioof potassium nitrate to sodium nitrate on a molar basis.

5. A method of treating a glass article consistent essentially of thefollowing components, in percent by weight, 3 to 6 percent LizO, 4 to 12percent of an alkali metal oxide selected from the group consisting ofNazO, K20 and mixtures of NazO and K2O, 2 to 12 percent P205, 15 to 28percent Al203, 40 to 60 percent SiOz, O to 3 percent ZnO, 0 to 4 percentMgO, 0 to 7 percent E203, and 0 to 4 percent ZrOz which comprisescontacting the glass at a temperature below the strain point with asource of at least two exchangeable alk-ali metal ions of differentionic diameter larger than lithium, the molar ratio of larger alkalimetal ions to smaller .alkali metal ions in said source being at least2:1, to exchange simultaneously at least two of the alkali metal ions ofthe source for 4alkali metal ions of the glass.

6. A method according to claim 5 in which the source of at leasttwoexchangeable different sized ionic diameter alkali metal ions oflarger ionic diameter than lithium ion is a molten mixture of potassiumand sodium salts.

7. A method according to claim 6 in which the mixture is a mixture ofpotassium nitrate and sodium nitrate.

8. A method according to claim 7 in which the mixture of potassiumnitrate and sodium nitrate is a mixture containing at least a 4:1 ratioof potassium nitrate to sodium nitrate on a molar basis.

9. A method of treating a glass article consisting essentially of thefollowing components, in percent by weight, 44.38 percent Si02, 26.61percent Al203, 5.04 percent LizO, 11.0 percent NazO, 9.96 percent P205,and 3.0 percent ZnO which comprises contacting the glass at atemperature below the strain point with a source of at least twoexchangeable alkali metal ions of different ionic diameter larger thanlithium, the molar ratio of larger alkali metal ions in said sourcebeing at least 2:1, to exchange simultaneously at least two of thealkali metal ions of the source for alkali metal ions of the glass.

10. A method of treating a glass article consisting essentially of thefollowing components, in percent by weight, 56.38 percent SiOz, 19.61percent A12O3, 5.04 percent Li20, 7.00 percent Na20, 2.96 percent P205,2.00 percent ZnO, 5.00 percent E203, and 2.00 percent Zr02 whichcomprises contacting the glass at a temperature below the strain pointwith a source of at least two exchangeable alkali metal ions ofdifferent ionic diameter larger than lithium, the molar ratio of largeralkali metal ions to smaller alkali metal ions in said source being atleast 2:1, to exchange simultaneously at least two of the DONALL H.SYLVESTER, Primary Examner.

k lk 1' t f al ah metal lons of the source for a a 1 me al rons o J OH NH H ARM ON, Assistant Examiner the glass.

References Cited UNITED STATES PATENTS 5 3,287,200 11/1966 Hess et al.161-1 3,357,876 12/1967 Rinehart 161-1 U.S. C1. X.R.

